Host defense against tumors might be augmented by enhancing the migration of macrophages into a tumor site. A reagent with the potential of producing a localized chemotactic response was prepared by covalently coupling rabbit anti-guinea pig hepatoma antibodies to the synthetic chemoattractrant formyl-methionyl-leucyl-phenylalanine (fMLP). The resulting complexes were chemotactic for guinea pig macrophages and reacted with surface antigens and the hepatoma cells. The in vivo effects of the antibody-fMLP complexes were assessed by their intraperitoneal administration to guinea pigs which were previously injected intraperitoneally or subcutaneously with hepatoma cells. The numbers of macrophages infiltrating the peripheral and central areas of the tumors received antibody-fMLP than in those injected with phosphate buffered saline, antibody or free fMLP. The mean tumor weights of the groups which were injected with antibody-fMLP complexes were lower than those of the other four groups although these differences were not statistically significant. These initial in vivo investigations demonstrate that antibody-chemotactic factor complexes can enhance the influx of macrophages into tumors, a response known to favor the host's tumoricidal defense mechanisms.